The invention relates to a method of straightening the supporting surfaces of supporting elements for optical elements in lens systems, in particular semiconductor lens systems.
To avoid deformations and achieve high precision of lens systems, the optical elements, in particular in lens systems of microlithography projection exposure equipment, are supported by elastic links, by which the optical element is mounted in a mount in such a way that it is isolated in terms of deformation. The elastic links serve in this case in particular for setting small differences in supporting force and consequently for low deformation of the optical element. The small differences in the supporting forces of the individual links are achieved by correspondingly great compliance.
It is disadvantageous in this case, however, that the elastic links are difficult to produce, i.e. that the supporting surfaces of the links are subject to tolerances in surface shape in such a way that they are at different heights. The known formula: supporting force=spring rigidityxc3x97spring excursion (F=Cxc3x97X) only allows the supporting force to be influenced. Since the spring rigidity of the elastic link cannot generally be further reduced for technical production reasons, only the spring excursion offers (as a compensation for tolerance in the surface shape) the possibility of minimizing the relative differences in supporting force or achieving a uniform spring excursion or deflection by the dead weight of the optical element.
The present invention is therefore based on the object of providing a method by which the supporting surfaces can be worked or aligned in such an accurate way that the differences in the positions of the supporting surfaces for the optical elements on the optical elements become as small as possible and consequently a low-deformation mounting is achieved.
This object is achieved according to the invention by the method steps stated in claim 1.
The use of a laser beam allows the production-related tolerances in the surface shape to be reworked much more accurately. This is achieved by the action of the laser on the supporting elements or the bearing part or parts causing bending, which according to the invention is controlled in such a way that the supporting surfaces for the optical element are subsequently located at least approximately all on the same supporting plane or within a very closely confined supporting range, with a very exact and low-deformation mounting being achieved.
One possible configuration in terms of the method may be that a so-called temperature gradient mechanism is used for the alignment, for the specific introduction of the direction of deformation and the amount of deformation of the elastic element. In the case of the temperature gradient mechanism (TGM), an asymmetric state of stress is established by local heating. The stresses occurring thereby locally exceed the flow limit and lead to plastic deformations and bending of the component toward the laser beam.
One advantage of the method according to the invention is also that, if need be, a repair can also be performed in this way, if for example a supporting element or an elastic spring arm has been bent.
With the method according to the invention it is also possible furthermore to use the laser alignment for isostatics, i.e. the spring arms are not directed at one plane but at a prescribed distribution, so that it is possible to compensate for deformations due to the dead weight, as is the case for example with a three-point mounting due to three-shaft sagging, at the seat or bearing point of the optical element.
It is also conceivable to carry out the method according to the invention under interferometric observation of the deflection of the cemented-in optical element and thus to counteract specifically existing deformations of the inserted optical element.
The use of a laser for straightening workpieces is generally already known. For example, EP 0 299 111 B1 discloses a method of straightening components which are out of true, such as axle journals, crankshafts, camshafts or similar workpieces, by surface hardening by means of a laser beam, with compressive internal stresses being introduced into the workpiece.
DE 198 05 849 also discloses the use of a laser for straightening or adjusting structural parts, for example optical components, which are to be interconnected. Proposed here as a connecting process is a joining technique in the form of soldering and adhesive bonding techniques, as well as welding by means of laser radiation. In this case, precision straightening by means of laser radiation is performed to adjust the structural parts, i.e. for positional changes or positioning operations.